BEIRUT – Syrian forces threatened Sunday to mount a “decisive battle” for Aleppo even as rebels clawed toward the city’s ancient center under intense bombardment and strafing from warplanes. In the capital Damascus, militiamen appeared to step up guerrilla-like forays in central districts that were once firmly in the regime’s hands.

The twin fronts reflected the rising stakes for both sides and a possible significant evolution in rebel strategies. Opposition forces appear to be shifting toward more hit-and-run strikes in Damascus and elsewhere to tie up Bashar Assad’s forces and blur the lines between rebel and government-held territory.

The biggest prize of the ambush brigades so far – 48 abducted Iranians branded as spies by rebels – was put on display in a video that carried a warning that all Iranians in Syria would be “captured or killed” because of Tehran’s strong backing for Assad. Iran said those captured when their bus was commandeered on Saturday were pilgrims visiting an important Shiite shrine on the outskirts of Damascus.

The abductions threaten to suck Iran deeper into Syria’s civil war and the wider political brinksmanship around the region. Iran claims it has no fighting forces aiding Assad, but it has sharply amplified its criticism of countries supporting the rebels such as neighboring Turkey and Gulf states led by Qatar and Saudi Arabia.

In further signs of the growing proxy nature of Syria’s conflict, Iran was forced to reach out to Turkey and Qatar with appeals to help return the captives.

The daylight hijacking on the main airport road in Damascus also raises questions about the regime’s ability to control key parts of the capital and protect the symbols of Assad’s power such as ministries and government offices– already breached once by a bombing last month that killed four members of Assad’s inner circle including his brother-in-law and the defense minister.

The pro-government Al Watan newspaper said the Syrian army is bracing itself for a “decisive battle” to clear Aleppo, Syria’s largest city, from rebels. It gave no possible timetable and for more than a week, activists have claimed the government is gearing up for an all-out offensive on the northern commercial center – another critical battleground for Assad’s regime to survive.

Government gunners pounded rebel-held areas to keep militiamen from expanding their hold on the city center, dominated by a medieval castle whose loss would be a deep symbolic blow for Assad. Local activist Mohammad Saeed said government warplanes joined in the attack by strafing rebel positions.

“Fighter jets to us are now as common as birds in the sky,” Saeed said.

Saeed and the British-based Syrian Observatory for Human Rights reported heavy shelling and clashes mainly in districts flanking Aleppo’s historic center: Salaheddine to the southwest and al-Sukkari and Hananou to the northeast. An amateur video posted on the Internet claimed to show shells crashing into a narrow street in Salaheddine, sending a shower of glass shards and chunks of rubble. The authenticity of the video could not be independently verified.

It’s clear, however, that Assad’s troops cannot simply surrender parts of Aleppo – a city of 3 million – to rebels without exhausting all military means. Rebels also have managed to keep open supply corridors from the Turkish border, about 25 miles (40 kilometers) away, which could also be targeted by any government ground offensive.

Syria’s official news agency SANA described the Aleppo rebels as “Gulf and Turkish militias” – more evidence of the deep regional fissures between Assad and allies such as Iran on one side and nations that back the rebels on the other.

The government also claimed it had regained full authority over Damascus after driving out rebels from central districts including near Abbassiyyin Square, a major roundabout. But residents reported loud explosions and gunfire echoing from several areas of the capital overnight and early Sunday.

Khaled al-Shami, an activist in Damascus, dismissed as “nonsense” the official reports that rebels were pushed from Damascus. He said rebels are increasingly using a tactic of quick-hit attacks to frustrate security forces and keep the capital unstable.

“The Free Syrian Army does not seek to hold territory in Damascus but rather stage hit-and-run attacks that drain the regime. The rebels are present and strong there,” he said.

U.S. Secretary of State Hillary Rodham Clinton plans to visit Istanbul later this week for talks with Turkish officials over the worsening crisis in Syria, the State Department said Sunday. Turkey has taken in tens of thousands of Syrian refugees and hosts some high-profile defectors from Assad’s regime.

Turkey’s state-run agency said Syria’s first man in space has fled to Turkey and joined opposition forces. The Anadolu agency said Mohammed Ahmed Faris crossed into Turkey after reaching Aleppo.

Anadolu said it was Faris’ fourth attempt to defect. The agency gave no other details on his escape and provided no source for the report. Faris, who was born in Aleppo in 1951, was part of a three-man crew of a Soviet space mission in 1987.

Turkey also has been drawn into the showdown over the Iranian captives with Tehran’s appeal for intervention.

A video broadcast by the pan-Arab network Al-Arabiya purported to show Syrian rebels guarding a group of Iranians. Armed men in the video identify themselves as members of the rebel “Baraa Brigades” and say that at least one of the 48 captives was an officer of Iran’s powerful Revolutionary Guards. They show what they say is his ID and a permit to carry weapons.

The captors claim the Iranians were on a “reconnaissance mission” in Damascus at the time they were abducted. Iran says they are pilgrims who were visiting a shrine to the granddaughter of the Prophet Muhammad.

“We promise Iran and all those who support this regime ... we will strike at all (Iranian) targets in Syria,” one of the rebels says in the video. “The fate of all Iranians who operate in Syria will be the same as those we have here, either captive or killed, God willing.”

In Tehran, a senior member of an influential parliamentary committee advised Iranians against traveling to Syria, state-run Press TV reported, in a high-ranking acknowledgment that Syrian rebels have expanded their hold over key roads and other areas once firmly under Assad’s control. The comment by Kazem Jalali, a member of Iran’s Committee on Security and Foreign Policy, does not represent an official Iranian discouraging travel to Syria, but the views of the parliament group often shape policies.